1. Field of the Invention
This invention generally relates to reflecting antennas for electromagnetic radiation and, more particularly, to reflecting antennas fabricated from fiberglass materials.
2. Description of the Prior Art
The most common type of electromagnetic reflecting antenna in use today is the spun aluminum dish. The spun aluminum dish is constructed from a disk-shaped, aluminum sheet that is pressed into a hollow shape or mold. The dish is formed by pressing an aluminum sheet against a rotating form or spinning chuck.
Although aluminum dish antennas are widely used today, these antennas have the problem of being subject to distortion and warpage. The aluminum is cold worked during the spinning process and therefore will distort if not handled carefully. In addition, aluminum is a very soft material and is easily dented by impact. Also, spun aluminum dish antennas are not easily produced to the close tolerances required in high frequency microwave applications.
Reflecting antennas are also manufactured from fiberglass and can be catagorized according to the process of fabrication. One such process is the technique of spray-up where the antenna is formed by the simultaneous deposition of glass fibers and resin into a mold to which a metalized reflecting surface has been applied. Fiberglass roving is fed through a mechanical chopper that cuts the filaments into short strips. The strips are thereafter fed into a resin-catalyst stream and deposited in the mold.
Although fiberglass antennas are easily constructed by this technique, sprayed-up fiberglass material is not as strong as comparably sized aluminum antennas. Accordingly, to achieve a strength comparable to aluminum, the sprayed-up fiberglass antennas typically have a substantially increased thickness and a correspondingly increased weight. Further, sprayed-up fiberglass antennas have poor surface accuracy because the polyester resin used in the fabrication process shrinks between 6% and 8% during curing. The shrinkage causes the surface to become distorted. In addition, after the antenna is molded, a portion of this shrinkage is stored as strain energy. Thus, the antenna can be easily deformed by subsequent impact.
Another process for manufacturing fiberglass antennas is the procedure of hand layup or contact molding wherein the woven roving is placed into the mold by hand. The resin and catalyst are also applied by hand. The air trapped in the mold is removed with rollers. Typically the layers of glass and resin are added one at a time to build up the cross section of the antenna to design thickness. Although the antennas fabricated by this method are of high strength, one problem with the process is that it entails high labor costs.
In both the aforedescribed spray-up and hand layup techniques the fiberglass antenna must have a metallicized surface applied to the fiberglass that will reflect electromagnetic radiation. Typically, this metallicized surface is applied using an aluminum flame spray process. In this process aluminum wire is heated to vaporization and thereafter sprayed upon the fiberglass surface. The vaporized aluminum solidifies into a conducting surface. Although this type of surface is easily obtained, it is difficult to achieve a uniform thickness across the surface of the antenna and long term adherence of the aluminum to the fiberglass.